1. Field of the Invention
The present invention relates to a lens sheet, and more particularly, to a lens sheet for both a microlens and a lenticular lens.
2. Description of the Related Art
Lens sheets are used in various fields. Representatively, lens sheets are applied to a liquid crystal display, a three-dimensional display, a surface light source device, a back light unit, a lens sheet for three-dimensional look, etc.
FIG. 1 is a view illustrating the structure of a conventional microlens sheet for three-dimensional look.
Referring to FIG. 1, a conventional microlens sheet 100 for three-dimensional look includes a lens array layer 110 in which convex lenses are arranged in arrays, a focal distance layer 120 which is formed under the lens array layer 110 and defines an appropriate focal distance in correspondence to the radius of curvature of the lenses, and a three-dimensional layer 130 which is formed under the focal distance layer 120 and on which a three-dimensional image is produced by predetermined three-dimensional patterns.
In such a microlens sheet, stereoscopic depth is realized by an observer's optical illusion, and a difference in depth is determined by the size and density of three-dimensional patterns. For example, in the case where 200 convex lenses are formed at an angle of 45° per inch, the observer feels protruding image volume if the number of three-dimensional patterns is determined to be less than 200 and retreating image volume if the number of three-dimensional patterns is determined to be more than 200.
In general, in the microlens sheet 300 for three-dimensional look, the shapes of lenses are formed by fabricating a mold using a method such as lithography and laser etching. In the case where the mold is fabricated through lithography, while a microlens of high quality can be manufactured, the size of lenses is substantially limited and cost becomes high. In the case where the mold is fabricated through laser etching, while a large-sized roll mold can be fabricated at a relatively cheap cost, the quality of the mold is likely to deteriorate and it is difficult to fabricate the mold in conformity with a design.
FIG. 2 is a view illustrating the structure of a conventional lenticular lens sheet.
Referring to FIG. 2, a conventional lenticular lens sheet 200 for three-dimensional look includes a lens array layer 210 in which a plurality of semicircular convex lenses are arranged in parallel, a focal distance layer 220 which is formed under the lens array layer 210 and defines an appropriate focal distance in correspondence to the radius of curvature of the lenses, and a three-dimensional layer 230 which is formed under the focal distance layer 220 and on which a three-dimensional image is produced.
Generally, in the lenticular lens sheet 200, while a mold of high quality can be fabricated using a diamond tool and image volume can be freely rendered, the image volume is limited depending upon an observing direction, and vertigo can be caused. Also, stable three-dimensional patterns cannot be realized using moire, unlike the case of the microlens.
Problems of the conventional microlens, which is manufactured by fabricating the mold through etching, are as follows.
First, the microlens by chemical etching is difficult to have a radius of curvature conforming to a design.
Second, the transparency of a lens sheet is degraded in proportion to a depth of embossings of the mold due to characteristics of chemical etching.
Third, when repeatedly manufacturing the same lens, it is difficult to uniformly fabricate the mold for a microlens in consideration of a state of an etching solution, an external temperature and an operator's condition due to the characteristics of chemical etching.
Fourth, while simple patterns can be realized to have a stable three-dimensional shape, realization of a three-dimensional shape of a general image is limited.
Problems of the conventional lenticular lens, which is manufactured by fabricating the mold using the diamond tool, are as follows.
First, while image volume can be freely realized in realizing a three-dimensional image, the image volume can be felt not in upward and downward directions but only in leftward and rightward directions.
Second, vertigo can be visually caused due to a jumping phenomenon of an image.
Third, three-dimensional realization of various patterns using a moire method becomes impossible.
In a convex lens three-dimensional printing sheet described in Korean Utility Model Registration No. 20-0311905 and a three-dimensional printing sheet described in Korean Utility Model Registration No. 20-0444099, a three-dimensional printing pattern image is expressed using a moire including multi-colored 2D images, and the conventional microlens is adopted.
A positive lens sheet of a flat surface described in Korean Patent Application No. 10-2006-0138521 relates to a flat three-dimensional sheet in which galipot with different refractive index is applied to surfaces of lenses such that the lenses are not viewed from an outside. The conventional microlens and lenticular lens are selectively used.
While various three-dimensional lens sheets which realize three-dimensional shapes by rendering engraved or embossed fine embossing patterns in place of three-dimensional printing are disclosed in the art, these lens sheets adopt microlenses of upward, downward, leftward and rightward radiation types.